High rates of sexually transmitted diseases (STD) among adolescents, especially African American adolescents, drive STD-related risk interventions. The current generation of interventions needs improvement. Understanding the relationship level determinants of STD risk may provide the tools needed to enhance future interventions. This study is focused on the associations among key STD risk factors: perceived risk for STDs (PRSTD), perceived partner concurrency, intimacy, and condom use. We have made significant progress in understanding how these constructs contribute to STD risk at the individual level, however, it remains to be seen whether relationship-level factors are exogenous to the individual. We propose that variation in partner characteristics in combination with relationship dynamics, such as trust and intimacy, contribute to the variance in STD risk within a girl. Further, as these STD risk factors vary by time in relationship, changes over time within a relationship must also be examined. The specific aims are (1) to determine changes over time within adolescent girls'sexual relationships in key STD-associated factors including perceptions of partner concurrency, intimacy, PRSTD, condom use, and STD acquisition;(2) to determine, within adolescent girls'sex partner relationships, associations among adolescent girls'perceptions of partner concurrency, intimacy, PRSTD, and condom use, and changes over time in these associations as relationships progress;and (3) to determine whether perceptions of partner concurrency and condom use predict acquisition and timing of incident STDs (gonorrhea and chlamydia) within adolescent girls'sexual relationships. We will recruit a new sample of 200 15-19 year old female patients attending either a general adolescent medicine clinic or an STD clinic in Baltimore, Maryland and follow them for 27 months. We will assess relationship characteristics, perceived partner concurrency, PRSTD, and sexual behaviors through daily electronic diaries and interviews every four months using PDAs. By obtaining weekly STD assessments, we will be able to establish stronger causal inferences about associations between condom use behaviors, and incident STDs. These data will inform scientists about factors which may guide biomedical HIV prevention strategies designed to interrupt transmission within a sexual relationship such as microbicides.